./configure --enable-ipv6 (requires getaddrinfo(3) and getnameinfo(3)).
TODO: make it work even without getaddrinfo(3) or getnameinfo(3)
(or, tcpdump/configure.in should provide alternative version by
AC_REPLACE_FUNCS)
TODO: make IPv6 filtering code work by default
TODO: make "protochain" friendly with optimization